Telegraph system



G. C. CUMMINGS.

TELEGRAPH SYSTEM.

APPLICATION FILED MAY 28, 1918.

v Patented Apr. 19, 1921.

//7 ven for George C Cumming/s &

y W Al /y.

G. C. CUMMINGS.

TELEGRAPH SYSTEM.

APPLICATION FILED MAY 28, 1918.

1,375,281. Patented Apr. 19, 1921.

2 SHEETSSHEET 2- /n ven/or Gea/ye C. Cum/Why s.

UNITED STATES PATENT OFFICE.

GEORGE C. CUMMINGS, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, 015 NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEGRAPH SYSTEM.

Specification of Letters Patent.

Patented Apr. 19, 1921.

Application filed May 28, 1918. Serial No. 237,009.

T 0 all ulzom 1'2 ma y concern:

Be it known that I. GEORGE C. CII'MMIXGS, a subject of the King of Great/Britain, residing at East Orange, in the county of Essex. State of New Jersey. have invented certain new and useful Improvements in Telegraph Systems, of which the following" is a full. clear, concise, and exact description.

This invention relates to telegraph systems and more particularly to systems for the simultaneous transmission of a plurality of messages over one and the same line circuit.

The principal object of this invention is to provide a system wherein superimposed alternating current and direct current telegraphic working may be carried on over the conductors of a telephone circuit or circuits.

In accordance with the present invention there is provided an improved arrangement of direct current duplex equipments and alternating current equipments combined to render independent telegraphic service over circuits or the conductor of one or more electrically balanced telephone line wire.

This invention is illustrated diagrammatically in the accompanying drawing, in which for the sake of clearness, only so much of a telegraphic and telephonic equipment has been shown as will be necessary to a clear understanding of the system.

Figure 1 of the drawing shows a combined direct current and alternating current telegraphic equipment suitable for use in the simultaneous transmission and reception of messages routed over one line conductor with an earth return circuit, such line conductor serving as one side of a metallic telephone circuit. arrangement of a combined direct current and alternating current equipment for rendering the same service as that possible with the equipment shown in the first figure. In Fig. 3 of the drawing, there are shown direct current duplex telegraphic equipments arranged to be operated in earth return relation from each conductor of a telephone line pair. while an arrangement for super.- imposing alternating current telegraph working is connected to operate in metallic circuit relation over the two line leads of the same telephone pair. In Fig. 4 there is shown a current amplifying and current filtering arrangement suitable for use with the alternating current telegraphic equipment Fig. 2 shows an alternate shown in the firstthree figures of the drawing. In Fig. 5 is shown a plan arrangement for applying the present system to communication service over two telephone line pairs; in this figure there is also shown an equipment for controlling the operation of telephone call signals over the commonly employed line conductors.

In describing this invention, a line lead 1 (Fig. 1) is connected to a direct current duplex equipment formed of a receiving relay 5, ratio arms 6 and T, and an artificial line AL. A transmitting branch is formed through a differentially connected impedance unit 9 and a transmitting-key 10, which controls the inclusion of oppositely poled line batteries 11 and 12 with the line lead. is well understood in the art. incoming direct current impulses over the line lead 1 may disturb the balance of this duplex equipment in such a manner that the armature of the relay 5 will be reciprocated in step with the reversals of the incoming impulses and in phase therewith establish connections from its correspondingly poled contact stops with a conductor 8, which may be understood as extending to a reading sounder or other suitable telegraphic receiving device (not shown). The conventional plus and minus signs atthe contact stops of the relay 5 will be understood to indicate that these stops are connected with correspondingly poled source of grounded current. In transmitting message impulses by means of direct currents. the key 10 may be operated to connect the batteries. 11 and 12 alternately through the impedance coil 9 to the apex or neutral point intermediate the bridge arm resistances 6 and 7. As well understood in the art, such impulses, although neutralized in the relay 5, nevertheless. become effective over the line lead 1 to operate a receiving relay at a remote station. For convenience, it may be assumed that an equipment identical to the equipment shown in the drawing is present at an assumed distant station.

In describing the operations by which alternating currents may be employed for the transmission and reception of messages over the line lead 1 at the same time that messages may be under transmission by the direct current method already described, a generator 15 of alternating currents, preferably of sine wave form is connected from operated position, a current path may be traced from the generator 15, by way of a resistance unit 16, the key contacts 19 and one winding 2Lof a current transformer 20, toearth. The alternating currents through the winding 21, induce corresponding currents in a second winding 22 of this transformer. These induced currents may comthence by way of two branching circuits, the first of which may be traced through a winding 26 of asecond transformer 25, a condenser 29, thence by way of the artificial line AL, to earth. The second path from the transformer may be traced through a second winding 27 of the transformer and a condenser 30, to the line lead 1. Since the two alternating current circuits just 7 as received, this magnet will operate the microphone at a frequency corresponding with the frequency of the alternating currents, which it will be understood is relatively high. In this manner the associated battery act-s on a transformer 37 ,to in turn operate a second similar amplifier comprising coil 38 and button 39 to induce currents in a filtering network comprising trans- 3 formers 41 and 42 and associated condensers.

After passing through th1s filtering neti work the amplified currents work a polarized relay &3 to interrupt a normallv closed cirplete' a circuit from an earth connection,

transformer coils 26 and 27 is differential,

one coil with the other, as relatesto the transmitted alternating currents just de scribed. Therefore, these currents are non effective with the third winding 28, and the receiving devices are therefore not affected by alternating currents transmitted from this station.

For the. reception of alternating current impulses incoming over the line lead 1, the ma or portlon of such alternating currents may traverse the condenser 30 (Fig. 1), the

, winding 27 of the transformer 25 and the winding 22 of the transformer 20, to earth. A portion of the incoming currents also traverses the winding 26 of the transform 1 25. the condenser 29 and the artificial line AL to earth. From the winding 27 and partially from the winding 26, correspond ing current changes are induced in the winding-28 of the transformer 25, and, turning to Fig. 4 of the drawing, since the winding of an amplifier control magnet 35 is connected from the terminals of the latter winding, thls magnet will correspondingly work a microphone pile which may be in the form of a telephone transmitter button. For concuit through a relay -14. The latter relay thereupon disconnects the winding of a polar relay =15 from a source of positive current and connects it to respond to a source of negative current, thereby disconnecting a telegraphic receiving device 16 which, for convenience, has been shown as a reading sounder. In this manner, the clevice 1L6 responds respectively to the presence and cessation of alternating current signals originating at a distance station.

The employment of current amplifiers as described in connection with the reception of alternating currents permits he use of relatively weak or attenuated currents of this character, particularly with respect to their strength in the line portion of the circuits. Such low wattage condition affords additional margin relative to the telephone, telephone call-signal and direct current telegraphic equipments not being affected by these alternating currents. In turn the current filtering equipment described, comprises such tuned or electrical time constant relation of circuits that current surges or extraneous electrical effects are screened out to incoming impul ses from the line lead 1 venience of description, the transformer 25 of Fig, 1 has also been included with the recelvlng equ pment shown in Fig. 4, In response to vlbratory or alternating currents as routed through the winding 22 of the transformer 20, the closed circuit through the winding 21 of this transformer provides a closed secondary therefor, and thereby improves the condition of the circuits for the reception of the alternating current signals.

intermediate the impedance coil 9 and the apex of the duplex equipment to earth is for the purpose of absorbing current surges which may otherwise interfere with the condenser 32, to earth, while the other branch extends through the winding 26 of the transformer 25, thence branching over two paths, one of which continues by way of the condenser 29 and the artificial line AL to earth, while the second path extends over the condenser 30 to theline lead 1. By means of the adjustable resistance 31 and the variable condenser 32, alternating currents transmitted by the key 17 as already described, may be rendered non-effective in the winding 28 of the transformer 26, due to equalizing differential current action through the windings 26 and 27 of the transformer 25. Therefore transmitted alternating currents established in sending by means of the key 17 are non-effective in the winding 28 and receiving equipment connected thereto which may be the same as the equipment shown and already described in connection with Fig. 4 of the drawing. For the reception of alternating current signals, such currents from the lead 1 traverse the condenser 30 and the winding 26 of the transformer 25, thence by way of the winding 22 of the transformer 20, to earth. Such currentsv through the winding 26 set up corresponding currents in the winding 28 of the transformer 25 to operate the receiving devices in.a manner corresponding with that already described.

In other respects the operation of the equipment shown in Fig. 2 corresponds substantially with the operation of the equipment shown and described in connection with the first figure of the drawing.

Referring to Fig. 3 of the drawing, two line conductors 60 and 61 forming a metallic circuit respectively terminate in telegraphic duplex equipments A and B, and are also provided with composite telephone equipment C. In this arrangement duplex equipments A and B correspondwith the direct current equipments shown in the first figures of the drawing. For thealternating current working, however, one set of telegraphic equipment has been dispensed with and the other alternating current equi ment arranged for operating in metallic circuit relation over the two line conductors 60 and 61. Since the direct current portion of the telegraphic equipment operates from the earth over the line conductors in the same manner as that already considered, no description thereof will be necessary in connection with the arrangement shown in this figure of the drawing. Therefore, description of the alternating current equipment will be proceeded with. In the transmission of signals from this point, the key 17 controls the inclusion of energy from the alternating current generator 15, through the winding 21 of the transformer 20, thereby setting up corresponding current changes in the winding 22 of this transformer, the terminals of which are connected through a local loop circuit which extends through the winding 27 of the transformer 25, the variable condenser 32, the adjustable resistance 31 and a second variable condenser 33. From one terminal of the winding 22, a branch circuit extends through the winding 26 of the transformer 25, thence branching over one path formed by the condenser 29 and the artificial line AL, to earth, and a second path formed through the condenser 30 and impedance unit 63 to the line conductor 61, lead 2. These currents reach the distant station and after passing through associated circuits at that point, reach the opposite line conductor 60 and return over an impedance unit 62, the lead 1, thence by way of the condenser 30 at the equipment A to a derived connection which reaches the opposite terminal of the winding 22 of the transformer 20. The connective relation of the windings 26 and 27 of the transformer 25 being differential, one with the other, the variable condensers 32 and 33 and the adjustable resistance 31 may be so proportioned that the strength of the transmitted alternating currents, traversing the winding 27. may in effect be equal to that portion of the alternating currents which traverse the winding 26 of this transformer. Therefore, such currents will not affect the winding 28 which may be connected to control receiving equipment simi lar to that already described in connectio with Fig. 4 of the drawing.

In receiving alternating current signals from the distant station, the impulses may be traced from the line conductor 60, impedance coil 62, lead 1, the condenser 30 (at A), winding 22 of the transformer 20, the winding 26 of the transformer 25, the condenser 30 (at B), line 2, impedance coil 63, thence returning to the distant station over the line conductor 61. These currents through the winding 26 of the transformer 25 setup corresponding current changes in the winding 28 of this transformer to shown in elemental form; it will be under stood, however, that in practice this network may be considerably elaborated upon with respect to additional equipment which may include inductance as well understood in the art.

It will, of course, be obvious that some portion of the alternating currents may be traced overthe bridge arms 6 and 7, condenser 13or perhaps incidentally over other paths formed in the general assembly of the equipment as shown. The proportion of the various capacities, resistances, etc., however, provides that such losses will be noneffective and relatively negligible, the work-' ing circuits being in accordance with those transmitting and receiving circuits which were defintely traced.

Referring to the telephone portion of the equipments, reference will first be made to Fig. 3 of the drawing. In tracing the circuits of the telephone equipment,.a path for telephonic currents extends from the line conductor 60 by way of a condenser 64, the'winding of an inductance coil 66, thence by way of a second condenser 68 to the oppositeline conductor 61. From the terminals of a second winding 67 of the induction coil 66, telephone equipment may be connected for communicating in metallic circuit relation over the line conductors 60 and 61. It will be understood that a conductor 69 connected at a central point on the winding 65 of the induction coil 66 may represent one side of a so-called phantom telephone circuit. One side of this circuit may operate in simplex or differential relation over thetwo line conductors 60 and 61, while the other side for this phantom circuit may be routed over a second telephone pair (not shown) in a manner similar to that in which the first lead of the phantom circuit is connected over the present line circuit pair. The condensers 70 and 71 and impedance coils ,7 2 and 73 as connected form a series parallel with the winding 65 of the induction or repeating coil 66 and a ground connection present intermediate the coils-72 and 73 further serves the purpose of absorbing or equalizing the current eifects with respect to the telephone equipment, while the condensers 74 and 75 respectively connected from each of the telegraph leads 1 and 2 to earth also serve in maintaining a suitable balance of the circuits and in preventing other than telephonic currents reaching h t epho e equipmen s.

Referring to Fig. 5 of the drawing, this plan arrangement provides that a sym metrically balanced telephone circuit A, formed of conductors 51 and 52, and a second similar circuit B, formed of conductors 53 and 5st, may be equipped with telegraphic devices similar to those shown and described in connection with Figs. 1 and 2 of the drawing and in addition thereto, these circuits may be provided with equipment for carrying on several difierent telephonic communications as over the transformer 66 for the lines A and B respectively, and by way of the leads 69 and 7 9 for phantom service over the metallic circuits. In connection with the circuit A there are shown equipments suitable for rendering service in the operation of call signals necessary, as well understood, in connection with the telephone service.

In initiating a telephone call signal by way of the jack 91 of the circuit A, it will be understood that equipment, not shown, but included with this jack is provided for applying an alternating current of relatively low frequency, as, say, approximately 16 cycles per second to the contact members 92 and 93 of'this jack. Such connection with the jack 91 separates its normal contacts to disconnect a signal receiving device 94 which, for convenience, is shown in the form of a drop. The low frequency alternating current traverses the contacts 86 of the relay 90 and conductor 85, thence by way of a condenser 100 and the windings of a polarized relay 101 to the conductor 95, thence returning over the contacts 87 of the relay 90 to the opposite side of the connection at' the jack 91. The armature of the relay 101 is so arranged thatits contacts 102 and 103 are normally opened. Under the action of 1 alternating currents through this relay, its

armature will be reciprocated in phase there-' with to alternately close the contacts 102 and 103. At the instant that the contacts 102 close, a source of grounded battery 104 I will be connected through the contacts 102 and associated resistances with a bus conductor 105. From this bus conductor, the circuit divides over one path formed through an alternating current relay 106 and a bat-V tery 107 to earth and over another path which includes the winding of a second alternating current relay 108 and a battery 109 to earth. While the battery 104 is con nected in differential relation with respect to the batteries 107 and 109', the potential. of the battery 104 is greater than the potential ofthe batteries 107 and 109. Ordinarily,

a relation in which the potential'of thebattery 104 is twice thepotential of each of the other batteries, has been found to give satisfactory results at the instant the-contacts 102 may be closed. It will, thereforefbe clear that current will flow from thebattery 104: 'over the circuits traced through each of the alternating current relays 106 and 108. Current in an opposite direction will flow from the batteries 10? and 109 through the respective alternating current relays during instants the contacts 103 of the relay 101 may be closed. In this manner, it will be clear that alternating currents will be active through the relays 106 and 108 during the time that alternating currents may be applied by way of the jack 91, as described, and it will further be clear that. the frequency of the alternating currents through these relays will be in phase. with the alternating currents applied at the jack. The relay 106 is of such design or adjustment that it will not be affected by the low frequency alternating currents, the relay 108,

however, being so proportioned or adjusted.

as to readily respond to the low frequency described. Operation of the relay 108 completes a circuit from a source of current 120 through the winding of a relay 110 to earth. Operation of the relay 110 completes circuit from a battery 120 which extends through the windings of a relay 111 and the relay 80 to earth. Relay 111 thereby operated completes a circuit which includes an alternat-.

ing current generator 112 through awinding 111 of transformer 113 and operation of the relay 80 causes separation of its contacts 81 and 82 to disconnect the conductois 85 and 95 and in closing itscontacts 83 and 84 connects a secondary winding 115 of the transformer 113 in a. closed loop circuit which includes a winding 67 of the transformer 66. The frequency of the current produced by the generator 112 is relatively high, say, approximately 133 cycles per second, and such currents may traverse the line conductors 1 and 2 of the metallic circuit A to reacha distant station and there operate a telephone call signal-which may be typical of the signal 94 shown at the'present station.

In describing operations whereby a telephone. call signal originating at a. distant station may be received at the present station, let it be assumed that at the distant point operations are carried out in a man ner similar to that just described in connection with the sending out of high frequency signalin currents from the present equipment. uch incoming alternating currents act through the transformer 66 to complete circuit over the contacts 81 and 82 of the relay 80, thence by way of the conductors 85 and 95 through the condenser 100 and relay 101.- The relay 101, is so constructed or adjusted that it may readily respond to alternating currents of either low or high frequency; therefore, in responding to the high frequency currents the contacts 102 and 103 of this relay are alternately closed in phase therewith to establish alternating curtery 116 to operate a relay 117m The operation of the latter relay connects the battery 116 to operate the relay 90, and separation of the contacts 86 and 87 of the latter relay disconnects the conductors 85 and 95 from the jack 91, while closure of the contacts 88 and 89 of this relay connects a. low frequency generator 118 to act over the normal contacts of the jack 91 in operating the dropf signal 94, it being understood, of course, that'no connection would be present in the jack 91during the time it will be necessary to send a. call signal, as described, from a distant station.

From the foregoing description it will be clear that by means of the present invention, a plurality of direct current channels and alternating current channels may be employed for the transmission of telegraphic messages during the time that one or more telephonic conversations may be under way on the same line conductors and further, that signaling'operations may be carried out on a telephone channel without interference with either the alternating current or the direct current telegraphic channels or with the other telephonic channels.

Although the present arrangement represents a preferred embodiment for carrying out the features of this invention, it will be understood that various changes in the organization of the apparatus may be made without departing from the spirit of this invention.

IVhat is claimed is:

1. In a system of simultaneous telephony and telegraphy, a circuit formed of a'pair of line conductors, means for telephoning over said circuit, duplex equipments for employing direct currents in signaling in earth-return relation over said line conductors respectively, and further equipment for each line conductor comprising a first transformer having several windings, a plurality of derived circuits opaque to direct currents extending from said respective duplex equipment to respective windings on said transformer, a second transformer connected to extend the derived circuits through said first transformer, and means included with said second transformer for employing alternating currents in telegraphing over said line conductors respectively independently of said telephoning and said direct current telegraphing.

2. In a telegraph s stem, a line circuit, means for employing direct currents in telegraphing over said line, means for employing alternating currents in telegraphing rents.

3. In a telegraph system, a lirl'e circuit, means for the transmission of signals over said line by means of alternating currents, a first relay responsive to each alternation of said currents, a second relay not capable of operating at a speed equal to the frequency of. said alternations controlled by said first relay. a third relay identical to said first relay controlled by said second relay, and telegrap iic receiving means controlled by said third relay.

4. In a system of simultaneous telephony and telegraphy. a plurality of circuits each formed of a pair of line conductors, means for telephoning over each of said circuits and for telephoning in phantom relation over said plurality of circuits, means for the control of telephone call-signals over each of said circuits and over said phantom circuit. earth-return equipment for each conductor of said circuits comprising du plex equipment for; telegraphing by means of direct currents, equipment for telegraphing by means of alternating currents over said line circuits, and a network com-- prising current amplifiers and current filters employed in the reception of said alternating currents,

5. In a telegraphsystem, a line circuit, equipment for the employment of direct currents in telegraphing over said line, a first transformer having several windings, condensers connected to form a plurality of derived circuits extending from different points on said direct current equipment to respective windings on said transformer, a second transformer connected to complete said derived circuits, a source of alternating currents, transmitting means to include said source of currents with said second transformer .in signaling over said line, impulse receivin means and means included with said first transformer to render said receiving means responsive to alterating currents from said line but unresponsive to .currents controlled by said transmitter.

6. In a telegraph system, a line conductor. duplex equipment including a real and an artificial line for the transmission and reception of signals by means of direct currents. a transformer having several windings. a condenser connected to form a derived circuit extending from said real line to a winding of said transformer, a second condenser connected to form a derived circuit extending from said artificial line to a second winding 'of said transformer, a sec- 0nd transformer, one winding ond transformer connected to connections through the first windings of said first transformer, a source of alternating currents, transmitting means for connecting said source of alternating currents through a second winding of the second transformer in the transmission of signals to said line circuit, impulse receiving means, a third winding 011 said first transformer non-responsive to said transmitted alternating currents included with said receiving means. and means controlled by said transmitting means for substituting a closed loop connection on the second winding of said second transformer during intervals of cessation of current from said extend said source of alternating currents.

T. In a telepgraph system,-a line circuit, duplex equipment including a real line, an artificial line and devices for the transmission and reception of signals by means of direct currents, a first transformer having several windings, a plurality of condensers, a derived circuit extending from said line circuit including a condenser and one winding of said first transformer, a second derived circuit extending from said artificial line including a condenser and another winding of said first transformer, a second transformer, a first winding of said second transformer connected to ointly extend the circuit of the two windings of the firsttransformerto earth, signal receiving means, a third winding for said first transformer for rendering alternating currents from said line effective with said signal receiving means, transmitting means, a second winding for said second transformer controlled by said transmitting means, and means controlled by said transmitting means for establishing a closed loop connection for said second winding of the second transformer during periods signals may be under reception by said receiving means. 1

8. In a telegraph system, duplex equipment including a real line, an artificial line and devices for the transmission and reception of signals conveyed by means of direct currents, a first transformer, a condenser connected in a first branch circuit extending from said real line through a first winding of said secand second 7 of said transformer. a condenser connected ings on said first and second transformers which includesa second winding of said first transformer and sa1d second artificial line, a source of alternating currents, means for former for said receiving means, and adjust- I able means in said second artificial line for conditioning said first transformer to render said transmitted currents non-effective with said alternating current receiving means.

9. In a telegraph system, a first and a second line conductor, duplex equipment including an artificial line and devices for the transmission and reception of signals by means of direct currents for each of said line conductors, a first and a second transformer included intermediate the duplex equipment of each of said line conductors, receiving means included with said first transformer, means included with said second transformer for the transmission of alternating currents serially over said first and second line conductors. a networki including capacity and resistance connected between said transformers. and means for employing telephones in communicating over said first and second line conductors. I

10, In a telegraph system, a plurality of line conductors. duplex equipment comprising an artificial line and devices for the reception and transmission of impulses by means of direct currents in ground relation over each of said .line conductors. a first transformer. a second transformer, a network comprising capacity and resistance to form a plurality of paths intermediate the duplex equipments ofsaid line conductors. means controlled by said first transformer for the reception of alternating current impulses, a source of alternating currents. transmitting means for rendering said alternating currents effective through said second transformer with said plurality of line conductors, means for employing telephones in communicating over said line conductors. and means controlled by said network for rendering said first transformer effective to neutralize the effect of alternating currents controlled by said transmitter with respect to said alternating current receiving means.

11. In a telegraph system, a first equipment comprisinga first and a second line conductor, duplex equipment including an artificial line' and devices for thereception.

and transmission of direct currentsignalmg impulses in earth return relation'over each of said line conductors, a derived circuit including two condensers extending from each of said line conductors to the associated artificial line, a first and a second transformer bridged at a point intermediate said condensers in joining the equipment of said first and second line conductors, alternating current receiving means included with said first transformer, a network comprising capacity and resistance included with said first transformer and the derived-circuit point intermediate said condensers of the equipment of the second lineconductor, a source of alternating currents, means for rendering said alternating currents effective with said second transformer in signaling in series over said first and second line conductors, means for telephoning over said line conductors, a second equipment typical of the first equipment described, and 1 means for providing additional telephone communications by way of the conductors common to the first and the second equipments.

12. In a communication system, a. metallic line circuit, means for employing-direct currents for telegraphing in ground relation over each conductor of said metallic circuit, means for employing telephones in communicating over said metallic circuit independent-1y of said direct current telegraphing, means for employing alternating currents in telegraphing in ground relation over each conductor of said metallic circuit independently of said telephoning and said direct current telegraphing, means for thereception of said alternating current telegraph signals comprising a first polar relay and a second polar relay, and a neutral relay intermediate said polar relays for establishing reversals of current polarity in controlling said second-mentioned polar relay.

13. In a system of composite telegraphy and telephony, a metallic line circuit formed of two electrically balanced conductors, means for providing a telephone channel for communicating over said metallic circuit, call signals for use in connection with said telephone channel. means for applying alternating currents over said metallic circuit in controlling said call signals, means for employing direct currents for telegraphing in earth return relation over each conductor of said metallic line circuit independently of said telephone communication channel and said call signal control, means for employ ing alternating currents in telegraphing in earth return relation over each conductor of said metallic circuit independently of said telephone channel, said call signal control and said direct current telegraphing, a first polar relay for the reception of alternating current impulses. a second relay normally energized through said first relay, a third relay of polarized type, normally disabld telegraph receiving means, and means controlled by said second-relay for rendering said third relay effective to energize said receiving means during periods alternating currents may be present in said first relay.

In witness whereof, I hereunto subscribe my name this 23rd day of May A. D., 1918. GEORGE C. CUMMINGS. 

